Photos

(FILES) This file photo taken on December 9, 2009 shows smog down a main street of Linfen, in China's Shanxi province, regarded as one of the cities with the worst air pollution in the world. An online documentary on China's notorious smog has become a viral sensation with more than 155 million views just one day after it was released, state media said on March 2, 2015. AFP PHOTO / FILES / Peter PARKSPETER PARKS/AFP/Getty Images